Antenna and communication device

ABSTRACT

An object of the present invention is to provide an antenna of small size which is excellent in the response characteristic in the high-frequency band, and to provide a communication device including the antenna having such an excellent characteristic that it is possible to realize downsizing of the device as a whole. That is, the present invention is directed to an antenna characterized by including a radiator made up of a carbon nanotube, and as a specific structure, for example, an antenna characterized by including an electrode that is connected with a part of the carbon nanotube and operates as a monopole antenna, and a communication device including the antenna.

TECHNICAL FIELD

The present invention relates to a novel antenna that enables thetransmission and reception of an electromagnetic wave, and atransmitting device, a receiving device, and a transmitting/receivingdevice each having the antenna.

BACKGROUND ART

In recent years, 2.5 GHz in the coming generation multimedia mobilecommunication and 2 to 30 GHz in a wireless LAN are employed in ahigh-frequency band in the communication field. Also, 5.8 GHz in an ETS(electronic toll system) and 76 GHz in an AHS (advanced cruise-assisthighway system) are used in a high-frequency band in an ITS (intelligenttransport system). In the future, it is expected that fields in whichthe high-frequency band is applied are further spread rapidly.

Also, a communication terminal represented by a cellular phone has beendownsized or brought into a built-in module, and the necessity of anantenna that is high in efficiency and small in size is rising inhigh-frequency receiving and transmitting portions adaptive to a broadband.

What are widely used as an antenna for a mobile radio device such as thecellular phone are a fixed helical antenna and a built-in type planarinverted-F antenna. In the fixed helical antenna, a fixed helicalantenna element is disposed to realize an antenna system that is smallin size and light in weight. Also, in the planar inverted-F antenna,radiation elements are so arranged as to be close to each other inparallel with a radio device substrate. A part of the radiation elementsis grounded to an earth point, and an electricity is fed to another partof the radiation elements from an electricity feeding point, to therebyrealize a short antenna and enable the design of a cellular phone inwhich the antenna is not projected from a cellular phone body.

However, even in both the cases of the fixed helical antenna and of theplanar inverted-F antenna, the communication band of the antenna elementis specified and therefore it is impossible to maintain the efficiencyover plural bands. Also, in the case where the communication terminal isbrought into a module, the size of the antenna portion becomes largerthan the size of a circuit chip, and the downsizing of the module isrestricted.

Accordingly, an object of the present invention is to provide an antennawhich is excellent in the response characteristic in a high-frequencyband and very small in size. Also, another object of the presentinvention is to provide a communication device that has a transmittingfunction, a receiving function, or a transmitting/receiving function,which includes an antenna that is excellent in the responsecharacteristic in the high-frequency band and very small in size, andwhich can realize the downsizing of the overall device.

DISCLOSURE OF THE INVENTION

The prevent inventors have found out that the above objects could beachieved by using a carbon nanotube that is excellent in a responsecharacteristic in a high-frequency band as a primary structural elementof an antenna, and have attained at the present invention.

That is, the present invention is directed to an antenna characterizedby including a radiator made up of a carbon nanotube, and as a specificstructure, for example, an antenna characterized by including anelectrode that is connected with a part of the carbon nanotube and isfor operating the antenna as a monopole antenna. The “monopole antenna”is directed to an antenna that has an electricity feeding portion (anantenna that is provided with an electrode) on an end portion of theradiator, whereas the “bipolar antenna” is directed to an antenna havingan electricity feeding portion in the center of the radiator.

A carbon structure represented by the carbon nanotube is broad in theabsorption wavelength band of an electromagnetic wave because of itselectron structure, and the electron structure of the carbon nanotubeapproaches one dimension. Therefore, it is presumed that the carriertransmission speed of the carbon structure is extremely high. As itsproof, it is found that in the carbon nanotube, a current-to-voltagecharacteristic does not comply with the Ohm's law, but exhibits aconduction mechanism called “ballistic conduction”. In the ballisticconduction, because a carrier such as an electron or a hole is merelyelastically scattered, the electric resistance is extremely low, and thecarrier transport can be realized at a high current density of 10⁹ A/m²or more. The present inventors have found out from the above phenomenonthat the carbon nanotube responds in the broad high-frequency band, andhave arrived at the present invention.

The antenna having the carbon nanotube as the primary structural elementin accordance with the present invention, as described above, not onlyexhibits absorption in the broad high-frequency band, but also cantransmit the absorbed high-frequency as a current at a high speed andwith a high efficiency because of the electric characteristic of thecarbon nanotube. Therefore the practicality is remarkably high as anantenna used for transmission and/or reception of the high-frequencyband. In the present invention, the “high frequency” generallycorresponds to a frequency having a concept as the high frequency, andmore particularly is mainly directed to a frequency band of from 100 MHzto 1 THz.

An antenna is usually so structured as to have the directivity in orderto efficiently transmit or receive an electromagnetic wave that ispropagated from one way or to one way by setting an element length of aradiation portion (also called “radiator” or “electricity feedingportion”, in both case of transmission and reception), which isconnected to an antenna line, to the length of about a fractional ratioof the wavelength (for example, λ/4, 3/8λ, λ/2, 5/8λ, etc.).

However, the antenna according to the present invention has an extremelyhigh electric conductivity as compared with a conventional antennamaterial because the radiator is made up of the carbon nanotube, and hasthe high conversion efficiency of the electromagnetic wave because theantenna remarkably absorbs a high-frequency signal. On the other hand,because the carbon nanotube per se is very small in size as comparedwith the wavelength of the electromagnetic wave, the directivity of theelectromagnetic wave in the propagating direction is low, and thesensitivity is high with respect to all directions as compared with theconventional material.

Accordingly, it is presumed that even if the directivity is not enhancedas in the antenna made of the conventional antenna material, arelatively excellent sensitivity (the electromagnetic wave conversionoutput of the degree that can be replaced by the antenna element of theconventional size) can be provided.

Also, since the carbon nanotube that is the primary structural elementsis microscopic, the antenna according to the present invention isastronomically small-sized as compared with the antennas of varioustypes which have been used as the antenna of the communication device(transmitting device, receiving device, or transmitting/receivingdevice) up to now, thereby being capable of manufacturing a remarkablysmall-sized communication device.

In addition, according to the present invention, since the carbonnanotube, which serves as an outlet/inlet for transmitting a highfrequency to the air and/or receiving the high frequency from the air,is remarkably short to the degree of about several hundreds μm at thelongest, the antenna according to the present invention as describedabove has substantially no directivity and is extremely preferable asthe antenna of the communicating device that is generally desired to benon-directive.

A structure of the antenna according to the present invention is, forexample, that one end portion or its periphery of the carbon nanotubethat serves as the radiator is connected to an electrode, and there maybe provided plural carbon nanotubes that are connected to the electrode.Also, at least a part of the carbon nanotube which is not connected tothe electrode may be fixed to another member (for example, anotherelectrode). In this case, when plural carbon nanotubes are provided, itis sufficient that a gap between the electrode and another member isbridged by at least a part of the plural carbon nanotubes, and bridgingmay not be made by all of the carbon nanotubes.

It is preferable that the carbon nanotube is a multi-wall carbonnanotube. When the radiating portion of the antenna is structured by themulti-wall carbon nanotube, it becomes easy to handle, and themanufacture efficiency is improved. In addition, because the respectivelayers function as parallel electric transmission paths, the conversionefficiency of the electromagnetic wave is improved, and the receivingsensitivity as the antenna is improved.

It is preferable that the diameter of the carbon nanotube is set to 0.3nm to 100 nm, and the length of the carbon nanotube is set to 0.1 μm to100 μm.

Further, it is preferable that the electrode contains any one ofmaterials selected from the group consisting of Au, Pt, Ag, Cu, Ta, Nband Ti.

It is preferable that the connection resistance between the carbonnanotube and the electrode connected with the carbon nanotube is set to10 MΩ or lower.

The electrode is generally disposed on the surface of the substrate. Itis preferable that a dielectric layer is formed on the uppermost surfaceof the substrate on which the electrode is disposed. Further, it ispreferable that the thickness of the dielectric layer is set to 1 nm to10 mm. It is preferable that the resistivity of the surface of thesubstrate on which the electrode is disposed to 1×10⁶ Ωcm or more.

It is preferable that at least a part of the carbon nanotube is coveredwith a protective layer. It is preferable that the protective layer is adielectric.

As the antenna of the present invention, a range of from 500 MHz to 1THz can be preferably set to the transmission band and/or receptionband.

The antenna of the present invention functions as a transmissionantenna, a reception antenna, or a transmission and reception antenna.

On the other hand, the communication device of the present invention isdirected to a communication device having a transmitting function(hereinafter, also called “transmitting device”) characterized byincluding the antenna of the present invention, a communication devicehaving a receiving function (hereinafter also called “receivingdevice”), and a communication device having a transmitting/receivingfunction (hereinafter, also called “transmitting/receiving device”).

The following examples are given as a transmitting/receiving device ofthe present invention.

1. The transmitting/receiving device, characterized by including areceiving circuit, a transmitting circuit, and a duplexer that changesover a circuit to be connected with the electrode of the antenna of thepresent invention between the receiving circuit and the transmittingcircuit.

2. A transmitting/receiving device characterized by including atransmitting circuit, an antenna of the present invention which isconnected to the transmitting circuit, a receiving circuit, and anantenna of the present invention which is connected to the receivingcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 are schematic perspective views showing three modes of anantenna according to the present invention, respectively, by using anexample in which one carbon nanotube is used;

FIG. 2 is a cross-sectional view of the antenna shown in FIG. 1( a),which draws the antenna more approximate to the original antenna thanthat shown in FIG. 1( a);

FIG. 3 is a cross-sectional view showing a state in which a protectivelayer is formed on the antenna shown in FIGS. 1( a) and 2;

FIG. 4 is a cross-sectional view showing a state in which a protectivelayer is formed on the antenna shown in FIG. 1( c);

FIG. 5 is a circuit diagram showing one example of the communicationdevice having a transmitting/receiving function in accordance with thepresent invention;

FIG. 6 is a circuit diagram showing another example of the communicationdevice having a transmitting/receiving function in accordance with thepresent invention;

FIG. 7 is an SEM photographic image (magnification of 20,000 times)showing the antenna in accordance with an example of the presentinvention;

FIG. 8 is a graph showing the transmission coefficient characteristic tothe frequency in the antenna shown in FIG. 7 in which the axis ofordinate is a transmission coefficient, and the axis of abscissa is afrequency;

FIG. 9 is an image obtained by subjecting a screen of an oscilloscopethat monitors an electromotive force when the antenna shown in FIG. 7 isirradiated with an electromagnetic wave, to image processing;

FIG. 10 is a graph schematically showing the image on a screen of theoscilloscope shown in FIG. 9;

FIG. 11 is an SEM photographic image (magnification of 3000 times)obtained by extracting only a portion at which a carbon nanotube islocated and enlarging the extracted portion in the antenna in accordancewith an example of the present invention;

FIG. 12 is a graph schematically showing an image on the screen of anoscilloscope that monitors an electromotive force when the antenna shownin FIG. 11 is irradiated with an electromagnetic wave; and

FIG. 13 is a graph schematically showing an image on the screen of anoscilloscope that monitors an electromotive force when an antenna of acomparative example using a high-orientation graphite is irradiated withan electromagnetic wave.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a description will be given in more detail of preferredembodiments of the present invention.

(Summary of the Invention)

FIGS. 1 are schematic perspective views showing three modes of anantenna according to the present invention, respectively, by using anexample in which only one carbon nanotube is used.

In FIG. 1( a), a pair of electrodes 12 a and 12 a′ are disposed on asurface of a substrate 14 a, and a carbon nanotube 10 a is so disposedas to have its end portions or their periphery connected to both of theelectrodes 12 a and 12 a′ and to bridge a gap between the electrodes 12a and 12 a′.

In FIG. 1( b), an electrode 12 b is disposed on a surface of a substrate14 b, and a carbon nanotube 10 b is so disposed as to have its one endportion or its periphery connected to the electrode 12 b and to projectthe other end portion of the carbon nanotube 10 b from an edge of theelectrode 12 b by about ⅔ exceeding the center portion of the carbonnanotube 10 b in the longitudinal direction.

In FIG. 1( c), an electrode 12 c is disposed on a surface of a substrate14 c, and a carbon nanotube 10 c is so disposed as to project verticallyfrom the substantial center of the electrode 12 c and to have one endportion thereof connected to the electrode 12 c. The present inventionmay be applied to any of the above cases.

It is preferable that each of the carbon nanotubes 10 a, 10 b and 10 cis covered with a protective layer formed for the purpose ofintercepting the carbon nanotube from the air and/or for the purpose ofphysically protecting the carbon nanotube.

Hereinafter, the respective structural elements of the present inventionwill be described.

(Carbon Nanotube)

In general, the “carbon nanotube” is directed to a graphen sheet ofhexangular net of carbon forming a tube in parallel with the axis of thetube. The carbon nanotube is further classified into a single-wallcarbon nanotube having a single-graphen structure, and a multi-wallcarbon nanotube having a multi-graphen structure. Which structure of thecarbon nanotube can be obtained is determined to a certain extentdepending on a composing method or condition.

In the present invention, the carbon nanotube which is the primarystructural element may be the single-wall carbon nanotube or themulti-wall carbon nanotube, but preferably the multi-wall carbonnanotube. When the radiating portion of the antenna is formed of themulti-wall carbon nanotube, it becomes easy to handle, and themanufacture efficiency is improved. In addition, because the respectivewalls function as the parallel electric transmission paths, theconversion efficiency of the electromagnetic wave is improved, and thereceiving sensitivity as the antenna is improved.

Also, a variety of carbon nanotubes such as a carbon nanohorn (a horntype that continuously expands the diameter from one end portion towardthe other end portion), a carbon nanocoil (a coil type that is spirallyshaped as a whole), carbon nanobeads (a shape in which a tube is providein the center and penetrates through a spherical bead made of amorphouscarbon or the like), a cap stack type nanotube, a carbon nanotube whoseouter periphery is covered with the carbon nanohorn or the amorphouscarbon can be also used as the carbon nanotube in the present inventioneven though they are not strictly tube-shaped.

In addition, a carbon nanotube containing some material therein, such asa metal contained nanotube in which metal or the like is contained inthe carbon nanotube, or a peapod nanotube containing fullerene or ametal contained fullerene therein can be also used as the carbonnanotube in the present invention.

As described above, in the present invention, in addition to the normalcarbon nanotubes, the carbon nanotubes of any figures such as theirvariety or the carbon nanotubes that are variously modified can be alsoused without any problems from the viewpoints of their electriccharacteristics and high-frequency characteristics. Therefore, the“carbon nanotube” in the present invention includes all of thosemodifications as its concept.

The composition of those carbon nanotubes can be conducted by theconventionally well known methods such as the arc electric dischargemethod, laser abrasion method, and CVD method, and any of the abovemethods can be used in the present invention. Among them, the arcelectric discharge method in the magnetic field is preferable from theviewpoint that a high-purity carbon nanotube can be composed.

It is preferable that the diameter of the carbon nanotube as used is setto 0.3 nm to 100 nm. When the diameter of the carbon nanotube exceedsthe above range, the composition becomes difficult, and it is notpreferable from the viewpoint of the costs. The more preferable upperlimit of the diameter of the carbon nanotube is 30 nm or less.

On the other hand, the lower limit of the diameter of the carbonnanotube is generally set to about 0.3 nm from the structural viewpoint.However, because the carbon nanotube that is extremely thin is notpreferable from the viewpoint that the yield at the time of compositionbecomes low, it is more preferable that the lower limit is set to 1 nmor more, and it is most preferable that the lower limit is set to 10 nmor more.

It is preferable that the length of the carbon nanotube as used is setto 0.1 μm to 100 μm. When the length of the carbon nanotube exceeds theabove range, composition becomes difficult or a specific process isrequired for composition, and it is not preferable from the viewpoint ofthe costs. When the length of the carbon nanotube is lower than theabove range, it is not preferable because it is difficult to connect thecarbon nanotube to the electrode. It is more preferable that the upperlimit of the length of the carbon nanotube is set to 10 μm or less, andthat the lower limit of the length of the carbon nanotube is set to 1 μmor more.

The number of carbon nanotubes that are connected to the electrode isone in the examples of FIGS. 1, and the antenna functions effectivelywith one carbon nanotube, but plural carbon nanotubes may also beapplied. In order to enhance the receiving performance and thetransmitting performance as the antenna, it is preferable that thenumber of carbon nanotubes is larger. The number of carbon nanotubes maybe appropriately selected while taking into consideration the downsizingor the manufacture costs of the antenna, and of the communicationdevice.

(Electrode)

An electrode that is one of the essential structural elements of thepresent invention may be formed of a pair of electrodes as indicated by12 a and 12 a′ in FIG. 1( a), or may be formed of one electrode asindicated by 12 b or 12 c in FIGS. 1( b) or 1(c), as described above.

In the present invention, the electrode is not limited if the electrodehas the conductivity, and the conventional materials can be employedwithout any problem, and it is preferable that the electrode includesany one of materials selected from the group consisting of Au, Pt, Ag,Cu, Ta, Nb and Ti. Each of those materials may be used by itself, but analloy consisting of two or more of those materials, or an alloyconsisting one or more of those materials and another metal may bepreferably used. Those materials are excellent in the conductivity andhigh in the machining property and the stability, and are used as theelectrode of the electronic device up to now.

In the present invention, when the electrode per se has the hardness toa certain extent and has the configuration retaining property, structuremay be made by only the electrode, but in general, the electrode isdisposed on a surface of an appropriate substrate as shown in FIGS. 1.

As shown in FIG. 1( a), in the case where a pair of electrodes 12 a and12 a′ are disposed on the surface of the substrate 14 a, it ispreferable that a gap between those electrodes (inter-electrodedistance) is set to 10 nm to 100 μm, and it is more preferable that thegap is set to 50 nm to 10 μm. When the inter-electrode distance is toolong, it is difficult to obtain a carbon nanotube that has a lengthsufficient to bridge the gap between the electrodes. Therefore, too longinter-electrode distance is not preferable. On the other hand, when theinter-electrode distance is too short, there is a case in which both ofthe electrodes are rendered substantially conductive, and themanufacturing of the electrodes becomes extremely difficult. Thus, theshort inter-electrode distance is not preferable from the viewpoint ofthe costs.

In the present invention, the thickness of the electrode is notparticularly limited. However, in the case where the electrode isdisposed on the surface of the substrate as in the modes shown in FIGS.1, the thickness is appropriately set to a range of from 10 nm to 100μm, and more preferably set to a range of from 50 nm to 1 μm.

In the present invention, it is not required that the electrode isdistinctly formed as the electrode as shown in FIGS. 1. For example, itis possible that a print wiring on a printed board of the communicationdevice is used as the electrode and connected with the carbon nanotube.Also, the carbon nanotube is connected to any member such as a lead wireor a frame, and those members may be used as the electrode.

(Relationship Between Carbon Nanotube and Electrode)

In the present invention, one end portion or its periphery of the carbonnanotube is connected to the electrode. Also, as shown in FIG. 1( a),the other end portion may be connected to another electrode.

In the present specification, the “one end or its periphery of thecarbon nanotube” means any halfway portion of the carbon nanotube in thelongitudinal direction from one end portion thereof, and “connected”means that at least a part thereof is connected, and it is unnecessarythat all portions thereof are connected. Also, in the presentspecification, “connect” means electric connection, and physicalconnection is not always required.

The same as that described above is applied to the structure in whichthe other end portion is connected to another electrode (for example,the structure shown in FIG. 1( a)).

A range of the positions of “the halfway portion of the carbon nanotubein the longitudinal direction” is not particularly limited. For example,even if most portions of the carbon nanotube in the longitudinaldirection are connected to the electrode (that is, even if the positionof “the halfway portion” is closer to the other end portion than to theone end to which the carbon nanotube is connected), or even if a portionof the carbon nanotube which is projected from the edge of the electrodeis slight or if there is no portion of the carbon nanotube which isprojected from the edge of the electrode, the present invention can beapplied. However, in the case where the entire carbon nanotube is hiddenin the electrode, the electromagnetic wave may be blocked. Therefore, itis preferable that the carbon nanotube is projected from the electrode.

It is preferable that a connection resistance between the carbonnanotube and the electrode connected with the carbon nanotube is set to10 MΩ or less, and it is more preferable that the connection resistanceis set to 1 MΩ or less. The excessively large connection resistance isnot preferable because the conductivity becomes insufficient, and thecarbon nanotube does not function as the antenna. Because the smallerconnection resistance is more preferable, the appropriate lower limitdoes not exist. However, in the case of the carbon nanotube and theelectrode, the lower limit is generally about 10 kΩ.

It is preferable that an angle defined between the carbon nanotube andthe electrode connected with the carbon nanotube is set to 10° or more.The angle is more preferably set to 30° or more, still more preferablyset to 45° or more, and most preferably set to the vertical. It ispreferable that the angle is close to the vertical in the case where thecarbon nanotube is disposed between two electrodes, because the carbonnanotube bridges the shortest distance between those two electrodes, andthe length of the carbon nanotube can be shortened.

In the present specification, the “angle” means an angle defined betweenthe carbon nanotube and the electrode at a portion where the carbonnanotube and the electrode are connected to each other.

The angle will be described assuming that a linear carbon nanotube and aplane electrode are disposed. In the case where one end portion of thecarbon nanotube is abutted against the plane of the electrode, and thecarbon nanotube is disposed in a direction normal to the plane of theelectrode or with a given angle with respect to the normal direction,the above angle means the smallest angle formed between the plane of theelectrode and the carbon nanotube. In FIG. 1( c), because the carbonnanotube 10 c is disposed in the normal direction of the electrode 12 c,the angle is 90° (vertical)

Also, in the case where a portion of the carbon nanotube extending froman end portion thereof to the halfway portion in the longitudinaldirection is linearly abutted against the plane of the electrode andprojected from the edge of the electrode, the above angle means thesmallest angle formed between the edge of the electrode and the carbonnanotube. In FIG. 1( b), because the carbon nanotube 10 b is projectedvertically from the edge of the electrode 12 b, the angle is 90°(vertical). The angle formed between the electrode 12 a or the electrode12 a′ and the carbon nanotube 10 a in FIG. 1( a) is also 90° (vertical).

In the case where the carbon nanotube is curved, or in the case where anon-linear carbon nanotube such as the carbon nanobeads is employed, forexample, it is difficult to obtain the angle in accordance with adistinct linear mutual relationship. In such cases, the above angle isobtained by drawing a tangent as required on a boundary between theabutting portion and the non-abutting portion of the carbon nanotubewith the electrode.

In the connection of the carbon nanotube with the electrode, when thecarbon nanotube and the electrode are linearly abutted against eachother as shown in FIG. 1( b), an adhesion of some extent can be expectedeven if the carbon nanotube and the electrode are not particularly fixedto each other. In the case where the firm connection is conducted, inthe case where the abutting portion of the carbon nanotube and theelectrode is short as shown in FIG. 1( a), or in the case where thecarbon nanotube and the electrode are in point contact with each otheras shown in FIG. 1( c), it is desirable to fix those members by somemethod. As a specific fixing method, although it is not particularlylimited, there is, for example, a method in which an electron ray isirradiated onto a portion to be fixed, to thereby deposit amorphouscarbon on the irradiated portion and fix the electrode and the carbonnanotube. Also, there is a method in which the carbon nanotube and theelectrode are fixed together during the manufacture of the carbonnanotube. Specifically, there is a method in which, during themanufacture of the carbon nanotube, the carbon nanotube is made to growdirectly on the electrode that also serves as a catalyst, or a catalystmetal is fixed on the electrode and the carbon nanotube is made to growon the electrode.

(Substrate)

In the present invention, as the substrate on which the electrode isformed as required, although not particularly limited, it is requiredthat at least the surface of the substrate on which the electrode isdisposed is of insulation. The specific resistivity of the surface ispreferably set to 1×10⁶ Ωcm or more, and more preferably set to 5×10⁶χcm or more. In the case where the resistivity of the surface is lowerthan 1×10⁶ Ωcm, the resistivity approaches substantially conductivity.This is not preferable because, for example, in the case of FIG. 1( a),insulation between the electrodes 12 a and 12 a′ cannot be ensured, andin the case of FIG. 1( b), the angle formed between the carbon nanotube10 b and the electrode 12 b becomes substantially 0°. On the other hand,although the upper limit of the resistivity of the surface is notlimited, the upper limit is generally about 1×¹² Ωcm.

The structure of a preferred substrate will be described with referenceto the antenna structured as shown in FIG. 1( a). FIG. 2 is across-sectional view of the antenna shown in FIG. 1( a), which draws theantenna more approximate to the original antenna than that shown in FIG.1( a). In FIG. 2, a substrate 14 a is structured in such a manner that adielectric layer 18 is formed on a surface of a support plate 16 on theside where the electrodes 12 a and 12 a′ are disposed.

The support plate 16 is formed of an Si substrate in this example.However, the present invention is not limited to this structure. Thethickness of the support plate is appropriately adjusted depending on amaterial to be used so as to provide a sufficient configurationretaining property, and is normally appropriately selected from the samerange as that of the normal electric wiring substrate.

The material of the dielectric layer 18 is SiO₂ in this example.However, the present invention is not limited to this. As the materialof the dielectric layer 18, it is sufficient to use materials by which athin film of dielectric is formed, and which is liable to ensure theadhesion with the electrodes 12 a, 12 a′, and the support plate 16.Examples of such materials are silicon oxide, silicon nitride, lithiumniobate, strontium titanate, or diamond.

With the above formation of the dielectric layer 18 on the uppermostsurface of the substrate 14 a, the electric insulativity can be given tothe substrate.

The thickness of the dielectric layer is preferably set to 1 nm to 10mm, and more preferably set to 10 nm to 1 mm. When the thickness of thedielectric layer is lower than 1 nm, there is a fear that the electricinsulativity is destroyed, and when the thickness of the dielectriclayer exceeds 10 mm, it is difficult to realize the downsized device.Thus, both of those cases are not preferable.

The various wiring boards disposed within the communication device canserve as a substrate on which the electrode of the antenna of thepresent invention is formed. In this case, it is preferable that theresistivity of the surface is within the above-mentioned range, and itis also preferable that the above-mentioned dielectric layer is formedon the uppermost layer.

(Protective Layer)

It is desirable that at least a part of the carbon nanotube is coveredwith the protective layer. FIG. 3 is a cross-sectional view showing astate in which the protective layer is formed on the antenna shown inFIGS. 1( a) and 2 in accordance with the present invention. In FIG. 3,the protective layer 20 a is so formed as to cover the entire electrodes12 a and 12 a′ and carbon nanotube 10 a.

The protection layer 20 a may be an electric conductor. However,preferably, it is a dielectric. Examples of the dielectric, which ispreferred as a protection layer from the viewpoint of air-blockingfunction or mechanical-protection function, include inorganic compoundssuch as silicon oxide, silicon nitride, silicon oxynitride, titaniumoxide, niobium oxide, lithium niobate, strontium titanate, and diamond;and various resins such as polyethylene, polypropyrene, polyvinylchloride, polyvinylidene chloride, an acrylic resin, a polycarbonateresin, a fluorine resin, an amide resin, polyethylene terephthalate,polyurethane, and polystyrene.

It is preferable that the protective layer is so formed as to cover theentire electrodes 12 a and 12 a′ and carbon nanotube 10 a as in aprotective layer 20 a shown in FIG. 3. However, it is sufficient that atleast a part of the carbon nanotube is covered with the protectivelayer. As long as part of the carbon nanotube is covered, it can beexpected that the covered portion is blocked from the air ormechanically protected. The blocking from the air is not required to beperfectly hermetically sealing. However, the completely hermeticallysealing is preferable, of course.

FIG. 4 is a cross-sectional view showing a state in which the protectivelayer is formed on the antenna shown in FIG. 1( c) in accordance withthe present invention. In FIG. 4, a protective layer 20 c is so formedas to cover the entire electrode 12 c and carbon nanotube 10 c. In theantenna shown in FIG. 1( c), it is relatively difficult to securely fixthe carbon nanotube. 10 c and the electrode 12 c, and there is a risk inthat the carbon nanotube 10 c is liable to drop off by means of anexternal force. However, the risk can be significantly reduced bycovering the entire carbon nanotube 10 c with the protective layer 20 c.

In the antenna having no substrate according to an example of thepresent invention, in order to provide the protective layer, it ispreferable to form the protective layer so as to cover the entire carbonnanotube from the connection portion of the electrode and the carbonnanotube, and it is more preferable that the protective layer is soformed as to cover the entire electrode in addition to the carbonnanotube.

The thickness of the protective layer is preferably set to a range offrom 100 nm to 0.1 mm generally, although the thickness is differentdepending on the material of the protective layer to be selected.

(Fabrication of the Antenna According to the Present Invention)

The above-mentioned method of fabricating the antennas of variousstructures in accordance with the present invention is not particularlylimited. Specific examples in which the electrode and the carbonnanotube are disposed on the substrate surface will be described below.However, the present invention is not limited to those examples.

A mask deposition method is simple and convenient as a method of formingthe electrode on the surface of the substrate. However, it is desirableto use an electron beam lithography method when the electrode is formedwith a higher precision, and especially when a pair of electrodes areformed and a gap between both of those electrodes should be morenarrowed.

In order to arrange the carbon nanotube that functions as the antenna onone electrode or between a pair of electrodes thus formed, there is amethod in which the carbon nanotube is disposed directly by using amanipulator while seeing it under a microscope such as a scanningelectron microscope (SEM). There is also a method in which the carbonnanotube is dispersed in an appropriate dispersion medium such asisopropyl alcohol or dimethylformamide, and the dispersion solution isdropped on one electrode or the edge thereof, or between a pair ofelectrodes and dried. In particular, in the case where the carbonnanotube is to be highly oriented between a pair of electrodes, there isa method in which after the dispersion solution is dropped between theelectrodes, an electric field is applied between the electrodes to alignthe carbon nanotube.

(Application of the Antenna of this Invention)

According to the present invention, a microscopic antenna can befabricated by just arranging the electrode and the carbon nanotube onthe circuit, and the antenna part as well as the communication devicesuch as a mobile portable terminal can be significantly downsized.

The antenna according to the present invention can set a broad bandincluding an extremely high frequency of from 500 MHz to 1 THz to thetransmission band and/or the reception band. In the antenna according tothe present invention, it is particularly preferable that particularly aband of from 800 MHz to 100 GHz is set to the transmission band and/orthe reception band.

The communication device according to the present invention is directedto the transmitting device, the receiving device and thetransmitting/receiving device characterized by including the antenna ofthe present invention which has the above-described excellentcharacteristics.

In the transmitting device according to the present invention, theelectrode in the antenna of the present invention is connected to thetransmission circuit, and a transmission signal is transmitted to thecarbon nanotube from the transmission circuit and released into theatmosphere as the electromagnetic wave (high frequency) In the casewhere a pair of electrodes (two electrodes) are disposed in the antennaof the present invention, the transmission circuit may be connected withonly one electrode or may be connected with both of the electrodes. Thesame is applied to a receiving circuit in the receiving device, aduplexer or a transmitting circuit and a receiving circuit in thetransmitting/receiving device as described below.

Specific transmitting devices may be a communicator, a radio microphone,a radio camera or the like.

In the receiving device according to the present invention, theelectrode in the antenna of the present invention is connected to thereceiving circuit, and a high frequency absorbed in the carbon nanotubefrom the atmosphere is transmitted as the reception signal to thereceiving circuit through the electrode.

Specific receiving devices may be a television, a radio, a radio clock,a GPS terminal represented by a car navigation system, a radio speaker,or the like.

On the contrary, the transmitting/receiving device includes thereceiving circuit and the transmitting circuit, and the antenna isconnected with the receiving circuit and the transmitting circuit. Inthe structure, in order to transmit the reception signal received by theantenna to the receiving device, and transmit the transmission signalfrom the transmitting circuit to the antenna, the transmitting/receivingdevice is generally equipped with a duplexer that automatically changesover the connection with the receiving device and the transmittingdevice. The transmitting/receiving device according to one embodiment ofthe present invention may include the duplexer. That is, thetransmitting/receiving device according to one embodiment of the presentinvention includes the receiving circuit, the transmitting circuit, andthe duplexer that changes over a circuit to be connected with theelectrode of the antenna between the receiving circuit and thetransmitting circuit.

FIG. 5 shows a circuit diagram of an example of thetransmitting/receiving device in accordance with this embodiment. InFIG. 5, reference numeral 34 denotes an antenna of the present inventionwhich includes a carbon nanotube 30 and an electrode 32, and theelectrode 32 is connected with a duplexer 36. The duplexer 36 has afunction of changing over a circuit to be connected with the electrode32 between the receiving circuit 40 and the transmitting circuit 50.

An electromagnetic wave (high frequency) absorbed by the antenna 34 issorted into the receiving circuit 40 as an electric signal by theduplexer 36. In the receiving circuit 40, after only a necessaryfrequency is extracted by a surface acoustic wave filter (SAW) 42 andamplified by an amplifier 44, the frequency is processed into a desiredsource such as a sound or an image and then outputted.

On the other hand, in the case where the transmission signal is sentfrom the transmitting circuit 50, the connection with the antenna 34changes over from the receiving circuit 40 to the transmitting circuit50 by the duplexer 36, and the transmission signal is released into theatmosphere as an electromagnetic wave (high frequency) by the antenna34. In the transmitting circuit 50, information inputted to an inputsection 56 is transmitted to a surface acoustic wave filter 52 as anelectric signal, only a high frequency necessary for transmission isextracted in the surface acoustic wave filer 52, amplified by anamplifier 54, and thereafter transmitted to the antenna 34 through theduplexer 36 as the transmission signal.

The above description is given of an example of thetransmitting/receiving device in which the duplexer is provided. Sincethe antenna according to the present invention is broad in the availablefrequency band, even if frequencies remarkably different between thetransmission signal and the reception signal are adopted, thefrequencies can be sufficiently dealt with by one antenna.

Incidentally, since the antenna according to the present invention ismicroscopic, one antenna is not so structured as to conduct bothtransmission and reception, but different antennas may be provided forthe transmitting circuit and the receiving circuit. That is, thetransmitting/receiving device according to another embodiment of thepresent invention includes the transmitting circuit, the antenna of thepresent invention which is connected to the transmitting circuit, thereceiving circuit, and the antenna of the present invention which isconnected to the receiving circuit.

FIG. 6 is a circuit diagram showing an example of thetransmitting/receiving device in accordance with this embodiment of thepresent invention. As shown in FIG. 6, the receiving circuit 40 isconnected with a reception-only antenna 34 a of the present inventionwhich is made up of a carbon nanotube 30 a and an electrode 32 a, andthe transmitting circuit 50 is connected with a transmission-onlyantenna 34 b of the present invention which is made up of a carbonnanotube 30 b and an electrode 32 b. The structures of the receivingcircuit 40 and the transmitting circuit 50 are identical with those inFIG. 5, and therefore the description thereof will be omitted.

In the transmitting/receiving device according to this embodiment,because it is not necessary to use the duplexer, the circuit structurebecomes simple, and therefore the manufacture costs can be reduced.Also, because the antenna of the present invention is microscopic, evenif dedicated antennas are provided for each of transmission andreception, independently, the transmitting/receiving device is stillsmall in size as a whole. Thus, the provision of dedicated antennas doesnot obstruct the downsizing of transmitting/receiving device. Inaddition, the conditions such as the configuration of the antenna, thenumber of carbon nanotubes, and the angle formed between the carbonnanotube and the electrode can be adapted to the respective functions oftransmission and reception. Therefore the performance of the overalltransmitting/receiving device can be improved.

Specific transmitting devices may be a cellular phone, a PHS, a basephone and a cordless handset of a cordless telephone, a radiotransceiver, or the like.

MORE SPECIFIC EXAMPLES

Next, the present invention will be explained in more detail by givingexamples. However, the present invention will not be limited by thefollowing examples.

First Example

(Fabrication of Antenna)

First, a pair of electrodes for connecting a multi-wall carbon nanotubewere formed on an SiO₂ layer side surface of an SiO₂/Si substrate (thethickness of an Si substrate as a support plate was 500 μm, and thethickness of an SiO₂ layer as a dielectric layer was 500 nm) so as toadjust a gap between those two electrodes to 2.5 μm. A mask depositionmethod was used as the forming method, and an electrode material as usedwas Au/Cr. The thickness of the electrodes was set to 50 nm.

One multi-wall carbon nanotube (40 nm in diameter and 4 μm in length)fabricated through the in-magnetic-field arc electric discharge processwas so arranged as to bridge the gap between the pair of electrodes thusobtained, and fixed. More specifically, a micromanipulator wasintroduced in an scanning electron microscope (SEM). The detailedprocedure will be described below.

First, a cathode deposit containing a high-purity multi-wall carbonnanotube that had been composed through the in-magnetic-field arcelectric discharge process, and the above-mentioned SiO₂/Si substrate onwhich the Au/Cr electrode was formed were inserted into the SEM. Onecarbon nanotube was picked up from the cathode deposit by using themicromanipulator, and was moved onto the surface of the SiO₂/Sisubstrate on which the Au/Cr electrode was formed, and was arrangedbetween the Au electrodes such that both ends of the carbon nanotube wasin contact with the electrodes. Thereafter, an electron beam (4×10⁻¹² A)was irradiated onto both ends of the multi-wall carbon nanotube forabout 1 minute so as to fix the multi-wall carbon nanotube to the Auelectrode.

The antenna according to this example was fabricated in theabove-mentioned manner. FIG. 7 is an SEM photographic image(magnification of 20,000 times) showing the antenna in accordance withthis example of the present invention. In FIG. 7, it is found thatrectangular shadows that are slightly visible on the upper left and thelower right are the pair of electrodes, and the carbon nanotube is soarranged as to bridge the gap between those electrodes.

(Frequency Characteristic)

The frequency characteristic of the antenna thus obtained according tothe first example was measured. More specifically, the transmissioncoefficient of the antenna according to the first example was measuredby a measurement system made up of a network analyzer (8753ES,manufactured by Agilent Technologies) and a coaxial air line. After themeasurement system was corrected, the antenna according to the firstexample was inserted into the coaxial air line, and the frequencycharacteristic was measured by using two ports. The measurementfrequency was 30 kHz to 6 GHz. FIG. 8 is a graph showing thetransmission coefficient characteristic to the frequency in the antennaaccording to the first example. It is found from the graph that in theantenna according to the first example, its transmittance rises fromabout 10⁶ Hz, and the transmission coefficient is hardly attenuated evenif the frequency is 6×10⁹ Hz.

(Electromotive Force)

The electromotive force induced when the electromagnetic wave of 800 MHzwas irradiated onto the antenna according to the first example wasmeasured. It was assumed that the transmission output of an oscillatorwas 800 mW, and a distance between the transmission antenna of theoscillator and the antenna of the first example was 20 mm. An inducedelectric power was monitored by an oscilloscope, and the electromotiveforce of about 150 mV was developed in this example in which themulti-wall carbon nanotube was used. FIG. 9 is an image obtained bysubjecting the screen of the oscilloscope at this time to imageprocessing as it is, and FIG. 10 is a graph schematically showing theimage. In the following example and comparative examples, only graphsschematically showing the image as required are attached.

An antenna was fabricated by using another multi-wall carbon nanotube inthe above-mentioned manner, and an experiment was conducted in the samemanner. Then, the electromotive force at that time was about 200 mV.

Second Example

(Fabrication of Antenna)

A pair of Au/Cr electrodes were fabricated on an SiO₂ layer side surfaceof the SiO₂/Si substrate with a gap of 10 μm therebetween as in thefirst example. The thickness of the electrodes was 50 nm.

The single-wall carbon nanotube composed through the arc electricdischarge method was purified (about 1 nm in the mean diameter), anddispersed in dimethylformamide at the ratio of 10 g/1. One droplet ofthe single-wall carbon nanotube was dropped in the gap between the pairof electrodes by using a micropipette and was then naturally dried.

The antenna of this example was fabricated in the above-manner. FIG. 11is an SEM photographic image (magnification of 3000 times) obtained byextracting only a portion at which a carbon nanotube is located andenlarging the extracted portion in the antenna in accordance with thisexample of the present invention. In FIG. 11, the single-wall carbonnanotubes seem to be white thin lines, and it is found that pluralsingle-wall carbon nanotubes are so arranged intricately as to bridge agap between the electrodes.

(Electromotive Force)

The electromotive force of the antenna according to the second examplewas measured in the same manner as that in the first example. Theelectromotive force of about 50 mV was developed in the antenna usingthe single-wall carbon nanotube according to this example. FIG. 12 is agraph schematically showing an image on the screen of the oscilloscopeat that time.

Because the single-wall carbon nanotube is microscopic, it is difficultto connect the single-wall carbon nanotube to the electrode,individually, and the connection resistance to the electrode is liableto rise. Also, since the single-wall carbon nanotube is structured byonly one wall, it is estimated that the sensitivity is lowered ascompared with the multi-wall carbon nanotube.

First Comparative Example

As a comparative measurement, an antenna made of high orientationgraphite was used as the antenna of a first comparative example. Theantenna has been conventionally used as the antenna of the generalcommunication device. The high-orientation graphite has graphiteoriented in a C-axial direction, and its size is 10 mm in length, 10 mmin width and 2 mm in thickness.

In the antenna according to the first comparative example, theelectromotive force was measured in the same manner as that in the firstexample. The electromotive force of about 100 mV was developed in theantenna of the first comparative example, using the high-orientationgraphite. FIG. 13 is a graph schematically showing the screen of theoscilloscope at that time.

Second Comparative Example

As a comparative measurement, an antenna obtained in the same manner asin the first example until the formation of the electrode, on which thecarbon nanotube was not disposed, was used as an antenna according thesecond comparative example.

In the antenna of the second comparative example, the electromotiveforce was measured in the same manner as that in the first example, butthe electromotive force was not observed.

(Consideration of the Results)

Taking the above into consideration, in the antenna according to thepresent invention in which the carbon nanotube is the primary structuralelement, it is apparent that the sufficient electromotive force isdeveloped by the electromagnetic wave, although the antenna ismicroscopic, and it is confirmed that the carbon nanotube functions asan antenna. On the contrary, in the first comparative example which isthe general antenna, although the sufficient electromotive force isdeveloped, the size of the antenna per se is extremely large, and thedownsizing of the communication device having the above antenna isobstructed. Also, in the second comparative example where no carbonnanotube is disposed, the electromotive force is not developed, and itis confirmed that the electrode as a single unit does not function as anantenna.

INDUSTRIAL APPLICABILITY OF THE INVENTION

According to the present invention, it is possible to provide an antennaof nano order which is excellent in the response characteristic in thehigh-frequency band, and the further high performance and downsizing ofthe antenna in the communication field are effectively large, and theindustrial usefulness is remarkably high.

Also, there can be provided a communication device that is provided withan antenna having the above-mentioned excellent characteristic and beingcapable of realizing downsizing.

1. An antenna characterized by comprising: a radiator formed of a carbonnanotube, an electrode that is connected with a part of the carbonnanotube and is for operating the antenna as a monopole antenna.
 2. Theantenna as claimed in claim 1, characterized in that the carbon nanotubeconnected to the electrode is disposed in plurality.
 3. The antenna asclaimed in claim 1, characterized in that at least a part of the carbonnanotube which is not connected to the electrode is fixed to anothermember.
 4. The antenna as claimed in claim 1, characterized in that theelectrode contains any one of materials selected from the groupconsisting of Au, Pt, Ag, Cu, Ta, Nb and Ti.
 5. The antenna as claimedin claim 1, characterized in that a connection resistance between thecarbon nanotube and the electrode connected with the carbon nanotube isset to 10 MΩ or lower.
 6. The antenna as claimed in claim 1,characterized in that the electrode is disposed on a surface of asubstrate.
 7. The antenna as claimed in claim 6, characterized in that aresistivity of the surface of the substrate on which the electrode isdisposed is 1×10⁶ Ωcm or more.
 8. The antenna as claimed in claim 6,characterized in that a dielectric layer is formed on an uppermostsurface of the substrate on which the electrode is disposed.
 9. Theantenna as claimed in claim 8, characterized in that a thickness of thedielectric layer is set to 1 nm to 10 mm.
 10. The antenna as claimed inclaim 1, wherein the electrode is connected with one end portion or aperiphery of the carbon nanotube.
 11. The antenna as claimed in claim10, characterized in that the carbon nanotube connected to the electrodeis disposed in plurality.
 12. The antenna as claimed in claim 10,characterized in that the electrode contains any one of materialsselected from the group consisting of Au, Pt, Ag, Cu, Ta, Nb and Ti. 13.The antenna as claimed in claim 10, characterized in that a connectionresistance between the carbon nanotube and the electrode connected withthe carbon nanotube is set to 10 MΩ or lower.
 14. The antenna as claimedin claim 10, characterized in that the electrode is disposed on asurface of a substrate.
 15. The antenna as claimed in claim 14,characterized in that a resistivity of the surface of the substrate onwhich the electrode is disposed is set to 1×10⁶ Ωcm or more.
 16. Theantenna as claimed in claim 14, characterized in that a dielectric layeris formed on an uppermost surface of the substrate on which theelectrode is disposed.
 17. The antenna as claimed in claim 16,characterized in that a thickness of the dielectric layer is set to 1 nmto 10 mm.
 18. The antenna as claimed in claim 1, characterized in thatthe carbon nanotube comprises a multi-wall carbon nanotube.
 19. Theantenna as claimed in claim 1, characterized in that a diameter of thecarbon nanotube is set to 0.3 nm to 100 nm.
 20. The antenna as claimedin claim 1, characterized in that a length of the carbon nanotube is setto 0.1 μm to 100 μm.
 21. The antenna as claimed in claim 1,characterized in that at least a part of the carbon nanotube is coveredwith a protective layer.
 22. The antenna as claimed in claim 21,characterized in that the protective layer comprises a dielectric. 23.The antenna as claimed in claim 1, characterized in that a transmissionband and/or a reception band is set to 500 MHz to 1 THz.
 24. The antennaas claimed in claim 1, characterized in that the antenna functions as atransmission antenna.
 25. A communication device having a transmittingfunction characterized by comprising the antenna as claimed in claim 24.26. The antenna as claimed in claim 1, characterized in that the antennafunctions as a reception antenna.
 27. A communication device having areceiving function characterized by comprising the antenna as claimed inclaim
 26. 28. The antenna as claimed in claim 1, characterized in thatthe antenna functions as a transmission and reception antenna.
 29. Acommunication device having a transmitting/receiving functioncharacterized by comprising the antenna as claimed in claim
 28. 30. Thecommunication device as claimed in claim 29, characterized by furthercomprising a transmitting circuit, a receiving circuit, and a duplexerthat changes over a circuit to be connected with the electrode of theantenna between the receiving circuit and the transmitting circuit. 31.A communication device having a transmitting/receiving functioncharacterized by comprising a transmitting circuit, a receiving circuit,and the antenna as claimed in claim 1 which is selectively connected toat least one of the transmitting circuit and the receiving circuit.